“This discovery marks a significant milestone, highlighting what only Hubble can accomplish,” said John Grunsfeld, associate administrator of NASA’s Science Mission Directorate, in a prepared statement.

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“In its 25 years in orbit, Hubble has made many scientific discoveries in our own solar system," Grunsfeld adds. "A deep ocean under the icy crust of Ganymede opens up further exciting possibilities for life beyond Earth.”

Ganymede, the largest moon in our solar system, is the only satellite with its own magnetic field. Its field generates aurorae that “rocks back and forth” when Jupiter’s own magnetic field changes.

Using NASA’s Hubble Space Telescope, a team of scientists at the University of Cologne monitored Ganymede's auroral belts and found that the amount of rocking in the magnetic field suggests that the moon has a subsurface saltwater ocean.

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"I was always brainstorming how we could use a telescope in other ways," project lead Joachim Saur, project lead, in a prepared statement. "Is there a way you could use a telescope to look inside a planetary body? Then I thought, the aurorae! Because aurorae are controlled by the magnetic field, if you observe the aurorae in an appropriate way, you learn something about the magnetic field. If you know the magnetic field, then you know something about the moon’s interior."